Water purification processes and potable water standards are improving all the time. However, in light of today's new water processing techniques and the knowledge of toxins, viruses, microorganisms, in particular contaminants of the aforementioned groups which are pathogenic for humans, and their ability to diversify, water processes and lines need to be monitored stringently. A lapse in the purification process or water pipe infrastructure could cause a significant spread of disease. With the projected increases for demand in water requirements, online monitoring systems for such contaminant are becoming more necessary.
For example, at present, microbiological quality control of the treated water is time consuming and inefficient. Like the chemical monitors in industry, pathogen monitors should respond on contamination, effecting an instant stop of the water purifying process on detection of contaminants. This will reduce the likelihood of any contaminant being transferred into the public domain.
Thus, it is important to assure that the water supplied to the general public by water utility boards is free of contaminants 24 hours a day. However, until today the time between sampling and response to a positive result (24 hours) is normally not sufficient to prevent contaminated water from reaching the public.
Different methods are used to detect contaminants. Detection of contaminants normally requires two general steps. The first step is the target capture, in which the contaminant of interest is removed, tagged or amplified to differentiate it from the remaining material in the sample. This step is typically responsible for the selectivity of the approach. The second step is the detection, in which the captured, tagged or amplified contaminant is counted or measured quantitatively. The detector typically acts as a transducer, translating the biological, physical, or chemical alteration into a measurable signal. In most cases where microorganism are to be detected, a third step, preconcentration, may be added prior to target capture because most liquids to be analyzed have relatively dilute levels of microorganism compared to other applications. For example, recreational water standards for bacterial indicators are roughly 100 cfu/100 ml, or 1 cell/ml. Since many detection methods are based on measuring less than a single cell, preconcentration may be necessary to achieve acceptable precision.
There are three broad classes of capture methods used in rapid detection technology which can be summarized as follows (Noble, R. T. and Weisberg, B.; 2005; Journal of Water and Health; vol. 03.4; p. 381). Firstly, molecular whole-cell and surface recognition methods capture and/or label the target compound by binding to molecular structures on the exterior surface or to structures within the interior of, a microorganism, virus, or to genetic material of interest. These include depending on the compound to be detected immunoassay techniques, bacteriophage, and molecule-specific probes, such as lipid or protein attachment-based approaches. Secondly, nucleic acid detection methods target specific nucleic acid sequences of bacteria, viruses, spores of bacteria or protozoa. These include polymerase chain reaction (PCR), reverse transcriptase polymerase chain reaction (RT-PCR), quantitative PCR (Q-PCR), nucleic acid sequence based amplification (NASBA), and microarrays. Thirdly, enzyme/substrate methods are based upon either existing chromogenic or fluorogenic substrate methods already in wide use, or new enzyme-substrate approaches. Enzyme/substrate methods are enhancements of currently approved methods such as the defined substrate technology employed in the commercial kits, Colilert® and Enterolert® (IDEXX Laboratories, Inc).
IDEXX kits are US EPA-approved tests, and are subsequently used as the standard water quality monitor for bacteria. The kits are cheap, reliable, sensitive, widely used, easily used and require little preparation of samples.
A disadvantage of the IDEXX kit as well as of most other methods referred to above is that it takes a long time to acquire results due to necessary sample/kit incubation periods. For example, the IDEXX kit takes between 18 hours to 25 hours to acquire results after sampling. This significant limitation prevents, for example, the use of this technique for rapid online applications. However, any new technology would need to be compared to IDEXX techniques owing to the fact that it is the technology currently employed as a standard internationally.
Different detection methods have different problems. For example, PCR technology and micro-arrays require the water samples taken to be treated with tedious enrichment and DNA extraction procedures, making exploitation of these techniques for online applications very difficult. Bacteria in environmental waters have also been monitored using flow cytometric techniques. In this case enrichment and cell preparations are usually necessary due to the low levels of poorly differentiated organisms normally present in such samples. Micro-fluidics as the name suggests are used to analyze small liquid volumes, when considering the large volumes of water purified in water processing plants this technique is obviously limited.
Thus, there is a high demand for detection methods which allow a sensitive but at the same time fast detection of contaminants.